This invention relates to a binder for use in the manufacture of very large sand shapes for the manufacture of large castings. Typically, so-called alkyd oil binders have been used in this application, because such binders provide relatively slow cure, as well as flexibility sufficient to facilitate separation of the sand shapes from the patterns from which, or in which, they are made. Furfuryl alcohol resins, on the other hand, have provided the basis of a large variety of highly successful foundry binders, and yet have not been used in any substantial amount in connection with the manufacture of extremely large sand shapes such as, for example, those used in the fabrication of slag pots for blast furnaces, and the like. The furfuryl alcohol resins have enjoyed widespread use, in part, because of the relatively rapid rate of cure which is achieved at room temperature using a relatively low amount of strong acid catalyst. However, in the manufacture of extremely large foundry sand shapes, the relatively rapid cure rate which is associated with furfuryl alcohol-based or furfuryl alcohol resin-based binders commonly would ordinarily lead to premature setting, or hardening of the sand mix and, in general, provides working lives too short to be practical. On the other hand, when catalyst levels are reduced sufficiently when using furfuryl alcohol-derived binders, to provide a working life of sufficient duration to accommodate the manufacture of the very large sand shapes, the rate of cure of the furfuryl alcohol binder is generally too slow to provide for a complete cure within the time-frame requirement imposed at the foundry and, in fact, it is insufficient catalyst for complete cure over any length of time.
On the other hand, the alkyd resin-based binders, though providing a slower cure rate, do suffer from several disadvantages. For example, the so-called "alkyd oil" binders are, in fact, mixtures of high hydroxyl alkyd resins, or a chemically modified highly unsaturated oil such as, for example, linseed oil, with polyisocyanate, and metal-ion catalysts such as cobalt naphthenate, stannous octoate, stannous naphthenate, and cobalt octoate. The hydroxyl moieties are derived from reaction of polyhydric alcohols, such as pentaerythritol and sorbitol with the oils. See U.S. Pat. Nos. 3,426,831 and 3,225,500 and British Pat. No. 1,073,843 for a discussion of the preparation of these binders. These complex mixtures do undergo some curing at room temperature. However, in order to achieve complete cure, heating is typically required. Even at room temperature, however, the alkyd binder-coated sands have a pronounced paint odor, and this odor becomes even more odious upon heating of the sand shapes during the curing process. This binder gives a greater preponderance of casting defects than the furfuryl alcohol binders. Mechanical methods of sand reclamation are not as effective on the alkyd oils as on the sand shapes made from furfuryl alcohol binders.
We have discovered that, in accordance with the present invention, a furfuryl alcohol-based resin, can be utilized in the fabrication of extremely large castings, using a binder which has all of the advantages of the so-called "alkyl oil" binder systems, with none of their disadvantages. Thus the furfuryl alcohol binders of this invention provide a slow rate of cure to permit time to fabricate along with high cured strength. Also they provide flexibility at strip to allow easy release from poor quality patterns. Furthermore, in accordance with the present invention, it has been observed that the furfuryl alcohol-based alkyd resin replacement binder provides a lower smoke evolution during pour, better reclamation of sand, and does not require baking for a complete and useful cure.